Photocatalytic materials such as TiO2 are used in many applications to obtain self-cleaning and air cleaning properties. The largest obstacle with photocatalytic materials is the scaling up to make large industrial productions. Ink jet printing technology is an economic way to apply functional materials using water-based suspensions or solvent-based suspensions. Ink jet printing is a non-contact deposition method, which can be used to obtain large area coverage with direct patterning on almost any substrate. The advantages of using ink jet printing are simplicity, low cost, less material waste, less environmental issues with spray aerosols and control of the coating.
Furthermore, materials and coatings with lasting performances, which are preserved over time, has been lacking. One drawback of using photocatalytic active TiO2 in for example building materials has been the lack of colour fastness and the change of colour upon light exposure. In the paper and the laminate industry TiO2 is an often used pigment but special grades of TiO2 with no or reduced photocatalytic activity are needed as photocatalytic TiO2 photogrey when exposed to light. Photogreying is an important quality property of pigment TiO2 used in the décor and paper industry and photogreying is of great practical importance because it can affect the colour of products such as paints, polymers, and cosmetics.
Photogreying is showing as the colour of TiO2 changes from white to dark violet upon light exposure. It has been suggested that photogreying is caused by reduction of TiO2 (probably from Ti4+ to Ti3+) during light irradiation in the absent of oxygen.
The process of photogreying can be explained by examining the photocatalytic properties of TiO2, which is shown in FIG. 1. When TiO2 is irradiated with light with a wavelength shorter than the band gap the absorbed photon can generate an electron/hole pair. Normally the electron travels in the conduction band to the surface where a reduction occurs. In most cases oxygen is reduced by the electron. In low oxygen environment, for example in a melamine formaldehyde resin matrix, the electron cannot be taken by oxygen and travels to the Ti-centre and creates Ti3+ centre. Ti3+ centres are purple/blue and create a blue toning of the product. This mechanism is known as photogreying. In for example laminates the high degree of polymerization and density of melamine formaldehyde resin makes the diffusion of oxygen and moisture from the surrounding environment very slowly and the oxidization of grey Ti3+ ions to the white Ti4+ ions becomes slow. However, the photo reduction of Ti4+ to Ti3+ is fast and thereby the laminate boards and panels become grey. Another important aspect with laminate boards and panels are the release of formaldehyde during curing. Formaldehyde is known to be a strong reduction agent and formaldehyde in the matrix can lower the overall partial pressure of oxygen and enhance photogreying. When the laminate boards and panels are stored in dark the photocatalytic reduction step is inhibited and within several days up to weeks the slow oxidation step turns the panels back to the original colour.
The photogreying process is reversible and oxygen is known to reverse the photogreying process but the change from dark violet colour to the original colour is much slower than the reverse reaction.
Within paper, décor paper, laminate flooring, laminate panels, foil and film industry photogreying is an important practical problem as the presence of cellulose and melamine formaldehyde resin enhances the photogreying of TiO2. Formaldehyde has been shown to enhance photogreying. For example in a melamine formaldehyde resin matrix in a laminate floor, the dark violet Ti3+ ions created by light exposure are relative stable as the partial pressure of oxygen is very low. The increasing Ti3+ concentration in the system results in greying of the product. Therefore, TiO2 grades for paper and laminates are surface modified as to be able to eliminate the greying. The TiO2 grades for laminates are surface coated to inhibit the photocatalytic cycle and thereby suppress photogreying of the products.
Various methods and techniques have been developed to overcome photogreying of TiO2 pigmented products. Common for all of these techniques are that photogreying is eliminated by inhibiting the photocatalytic process, and thereby inactivating the photocatalytic properties of TiO2.